1. Field of the Invention
The invention relates to control systems and more particularly relates to a pneumatic servo-controlled bellows actuator system.
2. Brief Description of the Prior Art
The present invention relates to bellows type pneumatic actuator systems. A rough schematic of such a system can be found in FIG. 1 of the drawings. As shown, it includes a pneumatically drivable bellows-type actuator and a servovalve which regulates the pressure of gas in the bellows. The bellows is provided with a position sensing device which has as an output a position signal corresponding to the position of the top surface of the bellows. This position signal is fed to drive electronics where it is compared with a control, or command signal to produce an output signal which corresponds to the difference of the position signal and the command signal. This output signal is the drive signal for the servovalve. Such a system is said to be "closed on position". In such systems the position of the controlled surface may be very precisely controlled.
Actuators of this type find numerous applications. In machining operations, for example, where it is frequently necessary or desirable to controllably move a tool and work piece with respect to one another in a precise manner, a bellows type actuatoring system may be used to provide that movement.
Another, demanding application for the bellows type actuator is in connection with G seats for vehicle simulators. G seats are vehicle seats which have been specially modified to provide controllable flesh pressure and other physiological stimulii to the operator of a vehicle simulator in order to duplicate the sensory cues which the operator would experience in the vehicle being simulated. These sensory cues are created by the motion of one or more surfaces which comprise the seat cushion and backrest of the G seat. These surfaces may be actuated by pneumatic bellows actuators. An example of such a G seat is disclosed in U.S. Pat. No. 3,983,640 entitled "Advanced G Seat for Aircraft Simulation".
Pneumatic bellows type actuators offer several advantages in these applications over other types of actuators, such as hydraulic actuators. Pneumatic actuators are safe, operating at a pressure of the order of 15 psi, and are also clean. They are reasonably maintenance free as well and are relatively low cost items, typically.
However, one problem with the pneumatic bellows actuator is its relatively poor response. That is, the intrinsic capacity for a bellows type actuator driven by a servovalve to quickly respond to a control signal and move to a desired position without overshoot and excessive transient movement is rather limited. In the absence of any feedback at all such systems have typical bandwidths of the order of less than 1 hertz. By closing the loop on pressure (providing pressure feedback) the system bandwidth can be extended to beyond 5 hertz, but this does not allow for accurate position control. By closing the loop on position (providing position feedback) and increasing the system gain, accurate control of position is possible, and bandwidth can be improved to something slightly over one hertz. But if the gain is further increased in an effort to improve responsiveness even more, it is found that such systems closed on position go unstable, that is, they go into self oscillation.
Nonetheless, it is considered desireable in the G seat area to have bandwidths of the order of 5 to 10 hertz. There thus exists a need to improve upon existing pneumatic bellows-type actuator systems closed on position to increase their bandwidth capability, and hence responsiveness.
Attempts have been made toward this end by the construction of electronic circuits which are intended to compensate for system characteristics which cause the aforementioned instabilities at higher gain levels. For example, one recent attempt included electronic feed forward compensation which extended the bandwidth of a pneumatic bellows servosystem to approximately 3 hertz with an 18 pound load. While this represents an advance over prior uncompensated systems it falls short of the aforementioned desired performance. In addition, while the bandwidth of that system for an 18 pound load was of the order of 3 hertz, the bandwidth decreased with increasing loads. In practice, the actual load requirements of such G seat actuators can be considerably higher than 18 pounds. An improvement is clearly needed.
The present invention is such an improvement. It provides electronic compensation for a bellows type pneumatic actuator system which extends the bandwidth of the system response to above 5 hertz. Significantly, it also renders the system bandwidth independent of load so that the system bandwidth remains broad even at the load requirements demanded for G seat operation. This important result represents a significant breakthrough in the area of bellows type pneumatic actuator systems.